The present invention relates to a single-vision aspherical lens used for correction of refractive power of human eyes. In particular, the present invention relates to a single-vision spectacle lens which is used when a wearer (a person wearing the spectacle lens) having an amplitude of accommodation views an object located at various distances raging from a short distance to a long distance.
When the wearer uses the spectacle lens, the wearer frequently uses an upper portion of the lens for distance vision and a lower portion of the lens for near vision. For this reason, it is desired that the upper portion of the spectacle lens has suitable performance for distance vision and that the lower portion of the lens has suitable performance for near vision.
Each of Japanese Patent Provisional Publications No. HEI 4-45419 and No. HEI 10-78566 discloses a spectacle lens designed to have suitable performance in the upper portion and the lower portion of the lens.
The spectacle lens disclosed in the publication HEI 4-45419 has one refractive surface configured such that a rate of change of curvature which is measured in an upper portion of the one refractive surface along a line extending on the surface from a center of the upper portion to the periphery of the upper portion is larger than a corresponding rate of change of curvature measured in the lower portion of the surface.
However, in each of four examples of the publication HEI 4-45419, only curvatures on two meridional lines (OA,OB) and a fact that curvatures on the other meridional lines take intermediate values between the curvatures of the lines OA and OB are described. Therefore, it is impossible for a person skilled in the art to practically make a spectacle lens based on the disclosure of the publication HEI 4-45419. That is, it is impossible to practically configure a spectacle lens having the suitable performance with regard to the upper portion and the lower portion thereof based on the disclosure of the publication HEI 4-45419.
Also, from graphs of astigmatism disclosed in each of the examples 2, 3 and 4 of the publication HEI 4-45419, it is understood that a minus lens disclosed in the examples 2, 3 and 4 is corrected for its astigmatism so that the astigmatism for the distance vision is substantially zero. When the astigmatism for the distance vision of the minus lens takes a value of zero, a dioptric power error of the minus lens takes positive values.
Further, in a case where the spectacle lens has a positive dioptric power error, a field of view of the wearer for the distance vision is blurred as if the wearer is in a fog. That is because it is impossible for the wearer to further decrease the refractive power of eyes for the distance vision due to the fact that the refractive power of eyes becomes lowest in the case of the distance vision, even though when the spectacle lens has a positive dioptric power error, an effect of the dioptric power error can be canceled by decreasing refractive power of eyes for the distance vision.
Accordingly, a balance between aberrations of the spectacle lens disclosed in the examples of the publication HEI 4-45419 is improper.
The spectacle lens disclosed in the publication HEI 10-78566 has a rotationally symmetrical asperical surface in which a center of the aspherical surface is shifted from a center of an outline of the lens in order to attain the suitable performance with regard to the upper portion and the lower portion of the spectacle lens.
However, the aspherical surface of the spectacle lens disclosed in each example of the publication HEI 10-78566 is a rotationally symmetrical surface for the purpose of easing the processing of the spectacle lens. Therefore, there is a limit on optical performance that the spectacle lens in the publication HEI 10-78566 can attains.